In recent years, the capacity of storage cells has increased, and power supply systems have been introduced to store power to be consumed by buildings, factories, stores, and private residences. By charging the storage cells (consuming power) in advance, these power supply systems can discharge power from the storage cells (supply power) at a predetermined time. In other words, by controlling the time at which the storage cells are charged and discharged, the time at which grid power (power supplied by a power company) is consumed can also be controlled.
The electrical utility rate for grid power usually includes both a fixed basic rate and a metered usage rate. Power companies set the basic rate so that the basic rate becomes less costly as the maximum amount of grid power consumed per unit of time becomes lower. The usage rate is also set so that the usage rate becomes less costly per unit of power at night, when less power is consumed, than during the day. Therefore, users of grid power can reduce the electrical utility rate they pay for grid power if they level out their consumption of grid power.
Therefore, a power supply system can suppress the rate for grid power by using grid power to charge storage cells during a period of time in which demand for power from users of grid power is low, or during the period of time in which the nighttime electric rate applies, and then discharging power from the storage cells, when the demand for power from users of grid power exceeds a predetermined threshold value, to compensate for the amount of demand exceeding the threshold value (the cross-hatched portion of FIG. 1).